The invention pertains generally to the field of bipolar transistor construction, and more particularly, to a bipolar transistor of extremely small dimensions using a polysilicon stringer base contact.
The frequency response and switching speed of bipolar transistors is related to the amount of parasitic capacitance that results from the transistor structure. In a bipolar transistor there is a junction capacitance associated with the emitter-base junction and another junction capacitance associated with the base-collector junction. There are also other junction capacitances such as the collector-substrate capacitance which affect the operation of the transistor. Junction capacitance is directly proportional to the area of the junction. Therefore, the smaller the area of the junction, the smaller will be the capacitance. Further, in bipolar transistors there is a base resistance which is made up of two components. The first component is called the extrinsic base resistance, and is the resistance of the path through the base region from the center of the base contact to the edge of the emitter region. The second component of the base resistance is the resistance of the path through the base region from the edge of the emitter to the center of the emitter, i.e., in the portion of the base region directly underlying the emitter region.
It has long been known that if the size of the base region can be reduced, and the width of the emitter can be reduced, then beneficial effects will occur to improve the performance of the transistor. One of these effects is that the size of the emitter-base and base-collector junctions will be reduced. This reduces the parasitic capacitance affecting these junctions. Further, if the size of the base region is reduced, the extrinsic portion of the base resistance will also be reduced by virtue of a shorter path length. Also, if the size of the emitter can be reduced, the intrinsic portion of the base resistance will also be reduced by virtue of a shorter path length from the edge of the emitter to the center of the emitter through the base region. Since reduced path length reduces the total resistance, the dimensional changes in the size of the base and emitter will result in lower parasitic capacitance and lower parasitic resistance values. The result will be a higher high frequency cutoff and a lower switching speed.
Thus a need has arisen for a bipolar transistor with base and emitter regions which are reduced in size.